1. Field of the Invention
This invention is directed to a process for making elastomer-based insulation for rocket motors and, in particular, to a process in which fragile carbon fibers are mixed with and preferably homogeneously dispersed in ethylene propylene diene monomer (EPDM) without requiring the use of a volatile solvent for dissolution of EPDM during fiber incorporation. The insulation of this invention is especially useful for placement in the nozzle or case, including between a solid propellant grain and a rocket motor case for protecting the case from high temperatures experienced during burning of solid propellant grains.
2. State of the Art
Solid rocket motors typically include an outer case or shell housing a solid propellant grain. The rocket motor case is conventionally manufactured from a rigid, yet durable, material such as steel or filament-wound composite. The propellant is housed within the case and is formulated from a composition designed to undergo combustion and thereby produce the requisite thrust for attaining rocket motor propulsion.
During operation, a heat insulating layer (insulation) protects the rocket motor case from heat and particle streams generated by the combusting propellant. Typically, the insulation is bonded to the inner surface of the case and is generally fabricated from a composition capable of withstanding the high temperature gases produced when the propellant grain bums. A liner layer (liner) functions to bond the propellant grain to the insulating layer and to any noninsulated portions of the case, as well as to inhibit interfacial burning. Liner compositions are generally known to those skilled in the art. An exemplary liner composition and process of applying the same to a case is disclosed in U.S. Pat. No. 5,767,221, the complete disclosure of which is incorporated herein by reference to the extent that it is compatible with this specification.
The combustion of solid rocket propellant generates extreme conditions within the case of the rocket motor. For example, temperatures inside the rocket motor case typically reach 2,760° C. (5,000° F.), and interior pressures may exceed 1,500 psi. These factors combine to create a high degree of turbulence within the rocket motor case. In addition, particles are typically entrained in the gases produced during propellant combustion. Under the turbulent environment, these entrained particles can erode the rocket motor insulation. If the insulating layer and liner are pierced during rocket motor operation, the casing is susceptible to melting or degradation, which can result in failure of the rocket motor. Thus, it is crucial that insulation compositions withstand the extreme conditions experienced during propellant combustion and protect the case from the burning propellant. It is also crucial that insulation compositions possess acceptable shelf life characteristics such that they remain sufficiently pliable, without becoming fully cured, until used in application to the rocket motor casing. This requirement is essential because the production of a given lot of insulation may have to wait in storage for a number of months prior to use. Typically, the insulation may be stored in large rolls in an uncured, or at most a partially cured, state until ready for use. A number of curing agents are well known and are conventionally employed but still must be compatible with the overall EPDM formulation to permit satisfactory shelf life. This in turn requires a balancing of curing agent activity.
In the past, attempts at producing insulating materials that would protect the rocket motor case focused on filled and unfilled rubbers and plastics, such as phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, polyester resins; and the like. These plastics, however, crack and/or blister as a result of the rapid temperature and pressure fluctuations experienced during combustion.
Elastomeric compositions have also been used as rocket motor insulation materials in a large number of rocket motors. The elastomeric compositions have been selected because their mechanical, thermal, and ablative properties are particularly suited for rocket motor applications. However, the ablative properties of elastomers are often inadequate for rocket motor operation. For example, insulation, whether thermosetting or thermoplastic, is characterized by relatively high erosion rates unless reinforced with a suitable filler. The criticality of avoiding such high erosion rates is demonstrated by the severity and magnitude of the risk of failure due to erosion. Most insulation is, of necessity, “man-rated” in the sense that a catastrophic failure can result in the loss of human life—whether the rocket motor is used as a booster for launch of a space shuttle or is carried tactically underneath the wing of an attack aircraft. The monetary cost of failure in satellite launches is well-publicized and can run into the hundreds of millions of dollars.
In order to improve the ablative properties of elastomeric compositions, it has been proposed to reinforce the elastomeric compositions with fillers, such as organic-based fibers or carbon fibers. For instance, an exemplary carbon fiber-filled rocket motor insulation composed of solid NORDEL® 1040 as the primary terpolymer is commonly known in the industry as the STW4-2868 thermal insulation and has the following composition:
TABLE ASTW4-2868 THERMAL INSULATION FORMULATION(carbon fiber; parts by weight)PartsIngredientFunctionby WeightNORDEL ® 1040Primary EPDM terpolymer base80Neoprene FBSecondary polymer base20Zinc oxideActivator5SulfurCurative1HAF carbon blackPigment1MBTAccelerator1AGERITE ® Resin DAntioxidant2AGERITE ® HPSAntioxidant1TelluracAccelerator0.50SulfadsAccelerator0.75VCM carbon fibersFiller41Total Parts by Weight153.25
Although organic-based fibers can be dispersed within the EPDM without too much difficulty, the homogeneous dispersion of carbon fibers in an elastomeric composition presents a difficult processing problem. The mixing process is complicated by the fragility of the carbon fibers. Mixing of carbon fibers into a solid elastomer under high shear physically deteriorates the carbon fibers into smaller particles or shreds, thereby negating the advantageous physical attributes that the carbon fibers would otherwise have contributed to the insulation.
Conventionally, the problem of carbon fiber fragility has been addressed by dissolving the elastomer into a solution with an appropriate organic solvent to lower the viscosity of the elastomer or elastomer mixture. Suitable solvents include, by way of example, hydrocarbons such as hexanes, heptanes, and/or cyclohexane. The frangible graphitized carbon fibers can then be mixed with the solution in, for example, a sigma-blade mixer without significant breakage of or damage to the carbon fibers. The material is then sheeted out and the solvent is allowed to evaporate at ambient atmosphere or in an oven.
The use of solvent in this processing technique presents several drawbacks. For example, solvent processing techniques, such as those conventionally used-to disperse carbon fibers in EPDM rubber, are relatively expensive. Material costs are increased by the use of solvents, as are processing costs, since additional workers and equipment are required to handle and process the solvents. Further, considerable costs and worker safety issues are associated with the disposal of hazardous volatile organic solvents.
Thus, although it has been long recognized that carbon fiber-filled EPDM is an excellent candidate for rocket motor insulation, a low cost and nonhazardous solvent-free synthesis route that produces EPDM insulation having carbon fibers homogeneously dispersed therein, but without being subject to significant breakage or damage would be desirable.